TW. 



580 



iiEPonT — 1884. 



onr data in this dlroction. The writer, stadyin*^ these results, fonnd 

 that the cylinder-condensation varied sensibly as the square i-oot of the 

 ratio of expansion, and this is apparently true for other forms and pro- 

 portions of engine. The amount of such condensation usually lies between 

 one-tenth and one-fifth the square root of that ratio, if estimated as a 

 fraction of the quantity of steam demanded by a similar engine having a 

 non-conducting cylinder. 



The state of tbe prevalent opinion on this subject, at the time of this 

 work of Clark and of Isherwood, is well expressed by the distinguished 

 German engineer, Dr. Albans, who, writing about 1840, says of the 

 choice of best ratio of expansion : ' Practical considerations form tbe 

 best guide^ and these are often left entirely out of view by mathematiciiins. 

 Many theoretical calculations ha* e been made to determine the point, 

 but they appear contradictory and nnsatisfactf^y.' Renwick, in 1848, 

 makes the ratio of initial divided by back pressure the proper ratio of 

 expansion, but correctly describes the effect of the steam-jacket, and 

 suggests that it may have peculiar value in expansive working, and that 

 tlie steam may receive heat from a cylinder thus kept at the temperature 

 of the ' prime ' steam. John Bourne, the earliest of now acknov/lcdged 

 authorities on the management and construction of the steam-engine, 

 pointed out, at a very early date, the fact of a restricted economic expan- 

 sion. Rankine "ccognised no such restriction as is here under considera- 

 tion, considered the ratio of expansion at maximum efficiency to be the 

 same as that stated by Carnot and by other early writers, and only perceived 

 its limitation by commercial considerations, a method of limitation of 

 great importance, but often of less practical effect than is the waste 

 by condensation. In his ' Life of Elder ' (1871), however, he indicates the 

 existence of a limit in practice, and places the fignro at that previously 

 given by Isherwood, for nnjacketed engines. By this latter date, the 

 subject had become so familiar to engineei's that a Avriter in ' London 

 Engineering,* in 1874, contemns writers who had neglected to observe 

 this limitation of efficiency as indulging in ' mediaeval twaddle.' 



A few writers on thermodynamics finally came to understand tbe 

 fact that such a limit.ition of applied theory existed. M. C. A. 

 Him, who, better than probably any authority of his time or earlier, 

 combined a knowledge of the scientific principles involved with practical 

 experience and experimental knowledge, in bis treatise on thermodynamics 

 (187G), concludes : '' QiCil est absolumcnt inipossihie d'edifier a priori we 

 theorie dc la machi)te a vapeur d'eau douce (Viui (•Jiaradcrc pcicnfiji'iur. ct 

 exact,' in consequence of the operation of the causes here detailed. While 

 working up his experiment upon the performance of engines, comparing 

 the volume of steum used with that of the cylinder, he had always found a 

 great excess, and had, at first, attributed it to the leakage of steam past 

 the piston ; but a suggestion of M. Leloutro set him upon the right 

 track, and he came to the same conclusion as had Watt so many years 

 before. He explains that errors of 130, or even up to 70 per cent, may 

 arise from the neglect of the consideration of this loss. Combes had 

 perceived the importance of this matter, and De Freminville suggested 

 the now familiar expedient of compression, on the return stroke as nearly 

 as possible to boiler pressure, as a good way to correct the evil. The 

 matter is now well understood by contemporary writers, and it has 

 become fully agreed, among theoretical writers as well as among prac- 

 titioners, that the benefit of extended expansion in real engines can only 



